Certain polymers, such as linear polydienes, exhibit cold flow at standard conditions. In other words, the polymers flow under their own weight, which causes problems when attempting to transport or store the polymers. Therefore, it is desirable to prevent cold flow from occurring by improving cold flow resistance of the polymer.
One solution employed in the art includes coupling the polymers. For example, linear polydienes, such as those polydienes produced by anionic polymerization or coordination catalysis, have been coupled with coupling agents. Although coupling agents may serve to improve cold flow resistance, they do not always serve to provide desired tire properties.
For example, in the art of making tires, particularly tire treads, functionalized polymers are advantageously employed to improve properties such as reduction in hysteresis loss. These functionalized polymers are often prepared by terminating living polymer chains with functionalizing agents that impart a functional group on the end of the polymer chain. These functional groups are believed to interact with filler particles in the rubber composition and thereby impart the desired reduction in hysteresis loss.
Unfortunately, the functionalizing agents employed to terminate the polymer and provide advantageous properties to the tire do not always serve to provide cold flow resistance to the polymer. Furthermore, the use of coupling agents limits the amount of chain end functionalizing agents that may be incorporated for improved tire properties.
Because functionalized polymers are desired, and in fact there is a desire for functionalized polymers with a high degree of functionality, the use of coupling agents that compete with the functionalizing agent can be detrimental to this goal. A need therefore exists to improve cold flow resistance by means other than coupling reactions.